1. Field of Invention
The invention relates to a so-called seat belt device with pretensioner for automatically retracting a seat belt. In particular, the invention relates to a seat belt device that retracts a seat belt as a precaution prior to a collision immediately after a vehicle collision is predicted, in order to reliably protect an occupant.
2. Description of Related Art
Numerous proposals have been offered related to pretensioner seat belt devices that automatically retract a seat belt during a vehicle collision, in order to protect an occupant with a greater degree of safety. For the pretensioner mechanism, devices have been widely known in which the seat belt is retracted at one stroke under great tension using an explosive or a spring, when the collision is detected.
In addition, recently, a number of suggestions have been put forward concerning pretensioner devices which prepare for a collision by retracting the seat belt as a precaution when a collision is predicted. In pretensioner devices that operate precautionarily in this manner, the seat belt is, for example, retracted at a predetermined tension by a motor, or the like. In seat belt devices adopting these pretensioner devices which retract the seat belt as a precaution prior to a predicted collision, the motor, or the like, is stopped and a normal state is returned to if the collision is avoided following prediction. Accordingly, the operation of such pretensioner devices is reversible, thus making them easy to use as compared to devices that utilize a nonreversible method such as an explosive, or the like.
A pretensioner seat belt device such as this, which retracts a seat belt as a precaution when a vehicle collision is predicted, is disclosed, for example, in Japanese Patent Laid-Open Publication 2001-247008. In the disclosed seat belt device, if a collision is predicted, a pretensioner device is operated while adjusting the tension at which the seat belt is retracted in accordance with the acceleration of the vehicle.
However, in this seat belt device, the seat belt is retracted with a fixed force (tension) if a collision is predicted, regardless of the state of the occupant within the vehicle.
Moreover, there is a high probability of a vehicle collision occurring following the prediction of a collision. However, it is possible to avoid some collisions through vehicle maneuvers by the driver. In these cases, if the driver is tightly restrained by the seat belt, a detrimental impact is exerted on the ability of the driver to execute breaking and steering operations. As a result, it is conceivable that some accidents occur which could have been avoided through driver vehicle maneuvers following prediction of the collision. In addition, there are also occasions when another occupant of the vehicle feels discomfort due to being excessively restrained by the respective seat belt.